1. Field of the Invention
This invention relates to milling tools for removing unwanted pipe such as casing, liners, tubing, drill pipe and the like from the bore hole of an oil, gas or mineral well. More particularly, the invention relates to a pilot casing mill which is characterized by a cylindrical pilot having helical vanes and adapted for mounting on one end of a cylindrical mill body fitted with multiple, removable and adjustable blades, each having multiple cutting elements on one edge thereof. The pilot and mill body are joined by threads and are designed to be extended into the bore hole on the end of a drill string and rotated with the drill string to effect the desired milling operation. In a most preferred embodiment of the invention the blades are removably and longitudinally adjustably mounted in spaced, longitudinal slots provided in the mill body, by means of wedge blocks and multiple cutting elements are mounted in cutting faces of the blades in staggered relationship to maximize cutting efficiency. The cutting elements are designed to seat in openings provided in the blades and to mill shavings, cuttings or "chips" of controlled size from the pipe or casing to be removed from the well bore. Each cutting element includes a shank mounted in one of the openings in the blades and an upper body segment extending from the shank above the blade surface. A cutting edge is provided on the top margin of the upper body segment and in a first preferred embodiment, a chip control groove is located adjacent to the cutting edge of round cutting elements.
The pilot casing mill of this invention is designed to cut the unwanted downhole pipe into "chips" or shavings of controlled size for easy removal by a suitable drilling fluid as the rotating cutting elements engage the pipe and mill it away. These shavings or "chips" are removed from the bore hole of the well by pumping the drilling fluid through the inside of the pilot casing mill, out the bottom of the bore hole and up the annulus of the bore hole to the surface. Lowering and rotation of the pilot casing mill is made possible using a conventional drill stem, column or string of pipe having sufficient length to reach the unwanted downhole pipe or similar obstruction, thus effecting its removal by milling it away. The pilot element of the pilot casing mill serves to centrally locate the cutting elements in the mill body radially around the uppermost end of the pipe or obstruction to be removed. The pilot has an outside diameter which is sized to closely fit the inside of the unwanted pipe, but not so large as to impede rotation or up and down movement of the pilot casing mill in the well bore. The grooves provided in the pilot between the helical vanes permit passage of the drilling fluid from the bottom of the bore hole, up the bore hole, to the surface. The blade elements of the pilot casing mill extend outwardly in radial fashion from the mill body a sufficient distance to insure coverage of the entire face of the downhole pipe to be milled or cut away. Rotation of the drill stem and pilot casing mill may be in either the clockwise or counterclockwise direction, but the device is normally designed for clockwise operation.
2. Description of the Prior Art
Conventional pilot mills are generally characterized by a one-piece construction where vanes are typically welded to the outside of a cylindrical body to form the pilot and blades are welded to the same body above the pilot in spaced, radially-disposed relationship. Cutting elements are then attached to the cutting faces of the blades by glueing, brazing or welding techniques and the tool is secured to a conventional drill stem or string of pipe of sufficient length to reach the unwanted pipe or obstruction and the pipe is milled from the well bore by rotation of the drill string and the pilot mill. In most cases, these cutting elements are composed of crushed cutting grade material combined with nonferrous metal at a temperature sufficiently hot to melt the nonferrous metal, thereby creating a brazing rod impregnated with carbide particles. This composite rod is then attached to the blades to complete the cutting tool. Use of tools having blades prepared in this manner results in an abrading, rather than cutting, of the unwanted downhole pipe or obstruction, and the penetration rate and tool life is therefore highly unpredictable. The resulting cuttings, shavings or "chips" removed from the downhole pipe are of random size and frequently interlock and jam in the well bore on their way to the surface in the drill fluid carrying medium.
Typical of the cutting tools and accessories known in the prior art is the "Composite Structure for Cutting Tools" detailed in U.S. Pat. No. 4,452,325, dated June 5, 1984, to Frederic J. Radd, et al. The structure includes a compact bundle of elongated, substantially cylindrical fibers or rods formed from a cemented metal carbide material. The fiber bundle is shrink-fitted within a metal collar to form a high quality cutting tool structure. A "Casing Mill" is detailed in U.S. Pat. No. 4,710,074, dated Dec. 1, 1987, to J. B. Springer. The casing mill includes a cylindrical body having one end adapted for attachment to a drill string and the other end fitted with pilot vanes which are added to guide the mill with respect to the casing. Multiple blades project outwardly from the body to present a cutting edge for milling casing upon rotation of the mill and polygonal tungsten carbide cutting elements are secured across a forward surface of each blade in a tesselating fashion, each element tilted in the direction of rotation at an angle between 10 and 20 degrees. To mount the cutting elements, the forward surface of the blade may include an inclined stair-step arrangement or inclined slots. Each element is brazed or bonded by a suitable organic adhesive to the forward blade surface. U.S. Pat. No. 4,478,296, dated Oct. 23, 1984, to Charles D. Richmond, Jr., details a "Drill Bit Having Multiple Drill Rod Impact Members". The drill bit is designed for drilling a subterranean bore hole and includes a housing having a cap plate adapted for connection to a drill pipe and a base block axially aligned with and spaced from the cap plate by guide members. Multiple, elongated drill rod impact members are carried by a drill rod holder plate, which is disposed between the cap plate and the base block, for movement longitudinally of the housing. The free ends of the drill rods extend through guide bores in the base block to define exposed working ends for chipping and crushing the formation upon rotation of the drill bit and introducing drilling fluid into the housing in a manner to effect a hammer-like action of the drill rods, which action progressively moves them longitudinally outwardly from the base block as they undergo wear. Each drill rod is preferably constructed in a cylindrical shape and is made harder at its center axis and progressively softer toward its outer periphery, so that the working ends of the drill rods are continually maintained in a generally pointed condition during operation. A "Milling Tool for Cutting Well Casing" is detailed in U.S. Pat. No. 4,796,709, dated Jan. 10, 1989, to Gerald D. Lynde, et al. The milling tool is designed to progressively cut away a section of casing installed within a well from the upper end of the casing. The milling tool includes multiple, elongated blades equally spaced from each other at intervals between one and three inches about the periphery of the cylindrical body of the milling tool. The blades are inclined with respect to the axis of rotation and carbide cutting discs, arranged in horizontal rows on the blades, form the inclined, leading planar face of the blades and the lowermost row of discs forms a cutting edge, with a negative rake engaging the upper end of the casing in the cutting operation.
It is an object of this invention to provide a pilot casing mill having removable and adjustable blades for milling unwanted pipe such as casing, liners, tubing, drill pipe and the like, from the bore hole of an oil and/or mineral well by attachment to a drill string.
Another object of the invention is to provide a pilot casing mill which is characterized by a pilot having helical vanes, a mill body attached to the pilot and multiple removable and adjustable blades mounted in longitudinal slots provided in the mill body for engaging the unwanted pipe and milling the pipe from the bore hole responsive to attachment of the opposite end of the mill body to a drill string and rotating the drill string.
Yet another object is to provide a pilot casing mill fitted with removable and longitudinally-adjustable cutting blades which receive and mount cutting elements in a manner which eliminates the necessity of welding or heat bonding and facilitates use of the cutting elements, which are seated in openings provided in the cutting blades and may be heat-sensitive or cannot be attached by welding or heat bonding techniques.
Still another object of this invention is to provide a new and improved pilot casing mill which is characterized by a pilot having helical vanes thereon for guiding the pilot casing mill into the bore hole of a well, a cylindrically-shaped mill body threaded on the pilot and provided with multiple, radially-spaced and longitudinally-oriented slots for removably mounting longitudinally-adjustable blades having cutting elements which engage the downhole pipe and mill the pipe from the well bore responsive to attachment of the mill body to a drill string and rotating the drill string and pilot casing mill in the bore hole.
Another object of this invention is to provide a pilot casing mill which includes adjustably and removably-mounted cutting blades provided with cutting elements of various design seated in openings arranged in alternating rows on each blade and alternating in sequence from blade-to-blade, to facilitate a more precise positioning of the cutting elements on the blades and greater precision in controlling the cutting element position on leading and following blades and the size of "chips" milled from downhole pipe by the cutting elements.
A still further object of this invention is to provide a new and improved pilot casing mill for milling unwanted pipe such as casing and the like, from the bore hole of a well, which pilot casing mill is characterized by a pilot having helical vanes for guiding the pilot casing mill into the bore hole; an elongated, cylindrically-shaped mill body threadably secured to the pilot and provided with radially-spaced, longitudinally disposed blade slots adapted for receiving removable and longitudinally-adjustable, radially-oriented blades; a split ring mounted in a split ring slot located in the mill body for receiving and threadably mounting multiple adjusting screws seated against the blades, respectively, for adjusting the blades longitudinally with respect to the mill body; and cutting elements seated in openings provided in the cutting face of the blades in staggered relationship for engaging and milling the unwanted pipe from the well bore responsive to attachment of the mill body to a drill string, rotating the drill string and cleanly severing each row of cutting elements from the blades to expose a new row of sharp cutting elements when the used cutting elements are dull.
Yet another object of this invention is to provide a pilot casing mill characterized by a pilot, a mill body attached to the pilot, multiple blades mounted by means of wedge blocks in longitudinal slots provided in the mill body and cutting elements of similar or dissimilar configuration mounted in a selected geometrical arrangement in openings or seats drilled or otherwise provided the cutting faces of the blades, which cutting elements each include a shank for engaging a corresponding seat in the blade and a flared or angulated upper body portion which terminates in a round, chevron or alternatively shaped cutting edge.